Cryogenic comminution system

ABSTRACT

A cryogenic comminution system is disclosed including a mixerconveyor and a comminution mill wherein cryogenic liquid refrigerant is introduced into the mixer-conveyor to embrittle the material to be comminuted before the material is fed into the mill; and wherein auxiliary cryogenic liquid refrigerant is separately introduced into the mill in order to overcome heat generated in the mill, and thereby maintain the internal components of the mill at a predetermined low temperature.

United States" Patent 11 1 Morita et a1., 241/65 Frable Nov. 13, 1973 [5CRYOGENIC COMMINUTION SYSTEM 3,658,259 4/1972 Ledergerber 241/23 xInventor: Norman B. Frame, Macungie Pa 2,583,697 1/1952 Hendry et a].241/17 X [73] Assignee: Air Products and Chemicals, Inc., PrimaryExaminer-Granville Y, Custer, Jr.

Allentown, Attorney-Ronald B. Sherer et a].

[22] Filed: June 17, 1971 I 21 Appl. No.: 154,113 [57] ABSTRACT Acryogenic comminution system is disclosed includ- [52] US. Cl. 241/65,241/23 mg a mlerfcopveyoi and a qmmmuuonfmn [51] Int Cl 1302c 21,00cryogenic liquid refrigerant is introduced into the mix- [58] d I17 2365 er-conveyor to embrittle the material to be commile 0 ea c nutedbefore the material is fed into the mill; and

1 wherein auxiliary cryogenic liquid refrigerant is sepa- [56]References Cited rately introduced into the mill in order to overcomeUNITED STATES PATENTS heat generated in the mill, and thereby maintainthe 2,879,005 3/1959 Jarvis 241/65 X internal components of the mill ata predetermined 2,609,150 9/1952 Bludeau.... 241/65 x low temperature3,614,001 10/1971 Beike 241/23 3,647,149 3/1972 2 Claims, 1 DrawingFigure PMENTEBNHV 1 3 m5 INVENTOR. Norman 5. Frab/e fwd 4: M

ATTORNEY I 1 CRYOGENIC COMMINUTION SYSTEM BACKGROUND OF THE INVENTIONThe present invention relates to the field of grinding, pulverizing orsize reduction, hereinafter collectively referred to as comminution.More particularly, the present invention relates to the comminution ofrelatively soft or resilient materials which are difficult or impossibleto comminute in high speed mills such as, for example, polymeric andrubber materials including polypropylene, polyethylene,polyvinylchlorides, polyisobutylenes and other natural or syntheticrubber materials, spices, pigments and color concentrates hereinaftercollectively referred to as relatively non-brittle materials.

With regard to such relatively non-brittle materials, it haslong beenrecognized that various refrigerants may be used to cool the materialwell below its'embrittlement temperature, and thereby comminutethematerial at low temperatures. However, previous systems have beenthermally and economically inefficient in that the material has beendeeply subcooled well below its embrittlement temperature so as toattempt to overcome the large amount of frictional heat which isgenerated in a high speed milLI-Iowever, the residence time of thematerial in contact with the mill isso short that the material must bevery deeply subcooled; i.e., lF to 150F or more below its embrittlementtemperature, in order to keep the mill sufficiently cold. Thus, thedeeply subcooled material; gives up only a small fraction of itsrefrigeration value to the mill, and is discharged at excessively lowtemperatures which is highly wasteful of costly refrigerant.

Attempts have also been made to introduce the refrigerant directly intothe mill, however, the short residence time of the material in the millproduces highly inefficient cooling of the material, which is equallywasteful of the costly refrigerant. That is, optimum speeds ofcomminuting can only be achieved when the material has had sufficienttime in contactwith the refrigerant so as to be embrittled before it iscomminuted.

SUMMARY OF THE NyENTIoN The present invention substantially reduces theamount of refrigerant bycarefully controlledcooling of the material to,or only slightly below, its embrittlement temperature in a cooling zone,and separately supplying a controlled amount of auxiliary refrigerantinto the mill to directly cool and maintain the internal components ofthe mill at an optimum operating-temperature. In the preferredembodiment of thepresent invention, the cooling zoneincludes arefrigerant injection zone, and a subsequent equilibration zone, wherebythe temperature gradient between the surface and core portions of theparticles is substantially reduced before they are fed into the mill.

BRIEF DESCRIPTION OF THE DRAWING The drawing is a simplifiedillustration of one embodiment of the cryogenic comminution systemshowing some of the components in cross-section.

DETAILED DESCRIPTION OF oNE PREFERRED EMBODIMENT 7 per 18 to theright-hand discharge end adjacent dis screw conveyors, or vibratoryconveyors so Ion g as they are capable of agitating the particles andconveying them through an elongated, closed chamber such as that formedby housing 24 which may be of circular or U-shapedcross-section. Inorder to reduce wasteful heat leak and consequent loss of refrigerant,the housing 24 is preferably covered with a thermal insulation material26, such as sponge rubber, foamed polyurethane, or other well knowninsulation materials. The mixer conveyormay be horizontal, or inclined,and may be in the order of 5 to 10 feet long. Shaft 12 is preferablydriven at a relatively low and variable speed by a motor 28, first gearreducer 30, variable speed pulley 32, belt 34, pulley 36, and a secondgear reducer 37..Thus, the rotational speed of mixer-conveyor 10 maybewidely varied, and is preferablyin the range of 5 to 10 RPM whichresults in a residence time of the material in mixerconveyor 10 in therange of l to 10 minutes for a mixerconveyor having the aboveindicatedlength of 5 to IOYfeet.

In order to cool the relatively non-brittle material 16 to a temperatureequal to or slightly below its embrittlement temperature, a liquefiedcryogenic refrigerant is stored in a conventional cryogenic storage tank38. As used in the present specification and claims, the term cryogenicrefrigerant is intended to denote a refrigerant fluid having a normalboiling point below minus 100F which includes, for example, liquefiedcarbon dioxide, liquefied air, various liquefied halogenatedhydrocarbons, liquefied nitrous and nitric oxide, and liquefied noblegases such as helium and argon. I-Iowever, for thermodynamic, safety andeconomic reasons, liquefied nitrogen having a normal boiling point ofminus'320F is preferred as the most ideal cryogenic refrigerant. p r Theliquefiedcryo genic refrigerant, suchas liquid nitrogen, is conveyedthrough insulatedtransfer lines 40,

42 ;and 44 to elongated sprayheader 46. Spray header '46 may comprise anelongated, pipe havingdischarge ports 48 spaced along its lengththrough'which. the cryogenic liquid refrigerant is injected into themixerconveyor. Alternatively, separate injection nozzles may be=spacedalong the mixer-conveyor so as to inject the cryogenic liquid intocontact with material 16. In either event, it hasbeen discovered thatfor optimumefficiency, one end of the liquid injection system shouldbepositioned immediately adjacent the product inlet end,

*and the points of liquid injection should extend toward the dischargeend for. one-half to four-fifths of the lengthof the'mixer-conveyor.That is, it is preferred that the cryogenic liquid injection portion ofthe mixer conveyor should terminate before the discharge end of materialshould'have a sufficient residence time in the vapor zone to enablethe-temperature gradient between thesurface and the core of theparticles to at least partially'equilibrate. Thus, the preferredmixer-conveyor includes a cryogenic liquid injection zone, and anequilibration zone so that the entire mass of the particle is cooled toits embrittlement temperature, or slightly therebelow.

As previously indicated, the discharge end of mixerconveyor is providedwith a discharge duct 20 through which the embrittled particulatematerial is discharged into the impact mill 50 which is illustrated asbeing of the conventional high-speed rotating hammer type. Of course,other types of well known comminution mills may be used such as, forexample, other types of rotary impact mills, pin mills, or ball millsdepending upon the type of particulate material and the final sizereduction desired. In any event, the feed inlets to such mills usuallyinclude either an inclined feed chute, or a feed conveyor which may beconnected to the discharge duct 20 of the mixer-conveyor eitherdirectly, or through an optional intermediate duct 22.

In the illustrated embodiment, the feed inlet to the mill comprises arelatively short feed conveyor 52 comprising a housing or duct 54containing a feed screw 56 which is driven by motor 58 through gearreducer 60, variable speed pulley 62, belt 64, and pulley 66. Whereas,the mixer-conveyor is relatively long and rotates at low speed in orderto provide a relatively long residence time in order to cool andembrittle the particulate material, feed conveyor 52 is relatively shortsuch as, for example, in the order of two to ten inches, and the feedscrew is driven at relatively high speed, such as in the order of 50 to100 RPM. As a result, the residence time of the particulate material indischarge ducts 20, 22 and the feed conveyor 52 is very short, such asfor example, in the order of one to ten seconds, and the volume of thefeed conveyor filled by the particulate material is less than one-halfof its total volume. Thus, the discharge ducts 20, 22 and high speed,partially filled, feed conveyor 52 provide an essentially open passagefor the conduction of auxiliary cryogenic refrigerant directly into themill as will now be described in detail.

Where an existing mill has such a feed conveyor, or an inclined feedchute, the preferred mode of auxiliary refrigerant injection is throughan auxiliary transfer line 68 the discharge end of which is preferablyprovided with a spray nozzle 70 located within the feed conveyor or feedchute. However, if the mill does not have a feed conveyor or feed chute,auxiliary refrigerant line 68 may terminate within discharge ducts 20 or22, or it may terminate directly within the mill itselfjthe latter twolocations being illustrated in phantom line. Thus, the point ofauxiliary refrigerant injection may be anywhere downstream of themixer-conveyor, so long as the auxiliary refrigerant does not havesufficient contact time with the particulate material so as toappreciably-subcool the particulate material, but rather, such thatthe'liquid refrigerant goes immediately into the mill and into directcontact with the internal components of the mill, Thus, the mill isdirectly cooled by the auxiliary liquid refrigerant and the heatgenerated in the mill is overcome without overcooling the particulatematerial substantially. below its embrittlement temperature.

Referring now to the refrigerant control system, main transfer line 42is provided with a flow control valve 72 which may be of the on-off ormodulating type, and which includes an actuator 74 for controlling theopening and closing movements of the valve. Actuator 74 is connected byan electrical or pneumatic signal line 76 to an indicator-controller 78which is connected through a signal line 80 to a temperature sensor 82,such as a thermocouple, which is preferably located in the mill outlet.Thus, sensor 82 detects the discharge temperature of the comminutedparticles, which is also an indication of the internal mill temperature,and indicator-controller 78 actuates controller valve 72 to increase ordecrease the flow of refrigerant through main line 42 so as to maintainthe discharge temperature of the comminuted material as warm as possiblewhile still maintaining optimum size reduction of the material. That is,depending upon the composition of the material and the final sizedesired, the discharge temperature is preferably maintained within 30Fabove or below its embrittlement temperature.

In order to properly proportion the amounts of refrigerant supplied tospray header 46 and auxiliary nozzle 70., refrigerant transfer line 44includes a flow control valve 84, and auxiliary refrigerant transferline 68 includes a flow control valve 86. A second temperature sensor 88is positioned in the discharge portion of the mixer-conveyor adjacentdischarge duct 20 and is connected via signal line 90 to a temperatureindicator 92. Thus, the temperature of the embrittled materialdischarged from the mixer-conveyor can be closely monitored and valve 84may be regulated so as to cool the material to, or slightly below, itsembrittlement temperature while also maintaining an optimum temperatureof the internal components of the mill by regulating valve 86.

While the size of flow control valve 72 is selected such that itsmaximum flowrate is more than sufficient to supply the required flowrateof cryogenic refrigerant during steady-state operation, a substantiallylarger flowrate is highly desirable during start-up when the completesystem is at ambient temperature and must be cooled-down to operatingtemperature; Accordingly, the preferred embodiment of the inventionprovides a by-pass line 94 having a manual by-pass valve 96 whichpermits higher refrigerant flowrates duringcooldown of the system. Inaddition, valve 96 permits a steady flowrate to be established in lines44 and 68, regardless of the action of temperature responsive valve 72,so that the optimum settings of proportioning valves 84 and 86 may beestablished. In this regard, extensive tests have indicated that betweenone-third to one-fifth of the total cryogenic refrigerant should beinjected through auxiliary line 68, and that optimum efficiencies formost materials are obtained by injecting approximately one-fourth of thetotal refrigerant through auxiliary line 68.

OPERATION In the preferred mode of operation, by-pass valve 96 is openedto permit a high flowrate of cryogenic liquid refrigerant to both sprayheader 46 and nozzle 70. Since the system is initially warm, asubstantial portion of the cryogenic liquid is initially vaporized incooling the lines, mixer-conveyor 10, and mill 50. Once all of thecomponents have been cooled down to a predetermined temperature, thematerial to be comminuted is introduced into hopper l6 and motors 28 and58 are energized. Temperature indicators 92 and 78 are monitored andvalves 84 and 86 are initially adjusted so'as to proportion the flow ofrefrigerant between spray header 46 and nozzle 70. By-pass valve 96 isthen closed, and the temperature indicators are further monitored as thesystem approaches steady-state operation. Final adjustments of valve 84and 86 are then made so that the temperature of the material at thedischarge end of the mixer-conveyor is at, or slightly below, theembrittlement temperature of the material to be comminuted. That is,valve 84 is adjusted such that the average equilibrated temperature ofthe material in the discharge end of the mixer-conveyor is held in therange of 0 to 50 below its embrittlement temperature; while valve 86 isadjusted so as to provide the optimum amount of additional refrigerantwhich is necessary to overcome the frictional heat generated in the milland keep the internal components of the mill at or near theembrittlement temperature of the material. Of course, it will beunderstood that the precise temperature of the material is notabsolutely determinable by sensors 88 and 82 since there is always amixture of material and vaporized refrigerant at the sensor locations.However, operating experience quickly enables the operator to determinethe relationship between the sensed temperatures and the actualtemperatures of the material and the mill. Also, it will be apparentthat the embrittlement temperature varies significantly with differentcompositions of material, and that the optimum residence time in themixer-conveyor is also dependent upon the initial size and mass of thematerial fed into hopper 16. Thus, the residence time of the material inthe mixer-conveyor may also be adjusted by varying the rotational speedof shaft 12 through variable speed pulley 32 which, in turn, optimizesthe amount of refrigerant supplied to spray header 46 in order tomaintain the average equilibrated' temperature of the materialdischarged from the mixer-conveyor between zero to 50F below theembrittlement temperature of the material being processed.

From the foregoing description it will be apparent that the presentinvention provides for carefully regu-,

lating the degree of cooling of the material to, or only slightly below,its embrittlement temperature, and thereby prevents significant waste ofrefrigerant which results from subcooling the material 100F to 150Fbelow its embrittlement temperature in order to overcome the heatingeffects of the mill. Thus, instead of attempting to rely upon theinefficient heat exchange between substantially subcooled material andthe mill components, the present invention separately injects auxiliaryrefrigerant into the mill for direct contact and heat exchange with thecomponents of the mill. As a result, extensive tests have shown that thepresent invention requires to percent less refrigerant,

which represents a substantial savings in operatingcosts as comparedwith all known prior systems.

From the foregoing description of one preferred embodiment of theinvention it will be readily apparent that numerous variations andmodifications will become readily apparent tothose skilled in the art,such as, for example, the substitution of an automaticindicator-controller for temperature indicator 92, whereby valve 84 maybe automatically varied in response to the temperature sensed by sensor88. Therefore, it is to be understood that the foregoing description isintended to be illustrative of the principles of the invention, and thatthe invention is not to be limited other than as expressly set forth inthe following claims.

I claim.

1. A cryogenic system for comminuting relatively non-brittle materialcomprising:

a. elongated rotary mixer-conveyor means including inlet means,discharge means spaced at least 5 to 10 feet from said inlet means, andvariable speed conveyor means for varying the residence time of materialconveyed through said mixer-conveyor within the range of one to tenminutes,

b. a source of liquid nitrogen,

c. first refrigerant injection means connected to said source forintroducing liquid nitrogen into said mixer-conveyor, said injectionmeans comprising a spray header positioned within the interior of saidmixer-conveyor and extending from adjacent said inlet means throughoutthe major portion of the length of said mixer-conveyor,

d. a comminution mill including feed inlet means connected to saidmixer-conveyor discharge means, and internal comminution means forcomminuting the embrittled material fed to said mill from saidmixer-conveyor,

e. temperature sensing means positioned in the discharge portion of saidmixer-conveyor for determining the temperature of the materialdischarged from said mixer-conveyor,

f. first flow control means connected to said first refrigerantinjection means for regulating the flowrate of liquid nitrogen suppliedto said mixerconveyor such as to maintain the temperature of thematerial discharged therefrom within a range of 0F to below 50F belowthe embrittlement temperature of said material,

g. second refrigerant injection means positioned downstream of saidmixer-conveyor for supplying additional liquid nitrogen into directliquid contact with said internal comminution means,

h. second temperature sensing means in said comminution mill fordetermining the temperature therein, and

i. second flow control means connected to said second refrigerantinjection means for separately regulating the flowrate of liquidnitrogen supplied to said mill such as to remove substantially all ofthe heat generated by said mill.

2. The cryogenic system as claimed in claim 1 wherein said comminutionfeed inlet means comprise a high-speed feed conveyor and said secondrefrigerant injection means are positioned such as to inject liquidnitrogen into and through said high-speed feed con veyor into directliquid contact with said internal comminution means.

1. A cryogenic system for comminuting relatively non-brittle materialcomprising: a. elongated rotary mixer-conveyor means including inletmeans, discharge means spaced at least 5 to 10 feet from said inletmeans, and variable speed conveyor means for varying the residence timeof material conveyed through said mixer-conveyor within the range of oneto ten minutes, b. a source of liquid nitrogen, c. first refrigerantinjection means connected to said source for introducing liquid nitrogeninto said mixer-conveyor, said injection means comprising a spray headerpositioned within the interior of said mixer-conveyor and extending fromadjacent said inlet means throughout the major portion of the length ofsaid mixer-conveyor, d. a comminution mill including feed inlet meansconnected to said mixer-conveyor discharge means, and internalcomminution means for comminuting the embrittled material fed to saidmill from said mixer-conveyor, e. temperature sensing means positionedin the discharge portion of said mixer-conveyor for determining thetemperature of the material discharged from said mixer-conveyor, f.first flow control means connected to said first refrigerant injectionmeans for regulating the flowrate of liquid nitrogen supplied to saidmixer-conveyor such as to maintain the temperature of the materialdischarged therefrom within a range of 0*F to below 50*F below theembrittlement temperature of said material, g. second refrigerantinjection means positioned downstream of said mixer-conveyor forsupplying additional liquid nitrogen into direct liquid contact withsaid internal comminution means, h. second temperature seNsing means insaid comminution mill for determining the temperature therein, and i.second flow control means connected to said second refrigerant injectionmeans for separately regulating the flowrate of liquid nitrogen suppliedto said mill such as to remove substantially all of the heat generatedby said mill.
 2. The cryogenic system as claimed in claim 1 wherein saidcomminution feed inlet means comprise a high-speed feed conveyor andsaid second refrigerant injection means are positioned such as to injectliquid nitrogen into and through said high-speed feed conveyor intodirect liquid contact with said internal comminution means.